But, but, the current best 'science' for storing hydrogen is 'hydrogen storage in metals'. It turns out that hydrogen is so small that it fits into metal crystalline lattice interstitials, and you can squeeze more of it into metal than there is in liquid hydrogen. Importantly, it then leaches out of metal so it is inherently safer than a high-pressure gas or cryogenic liquid.

The problem is with slow loading and how to combat contamination, and with the fact that you need expensive metals: it would be nice if it worked with iron or aluminum but nooo, hydrogen wants something like palladium. People are working on cheaper matrix materials, though.

The one word answer to your headline question might be fracking. If the reserves of gas that are alleged to be available from that recovery method, turn into reality, then in the future the US and many other countries will have that as their main and only low cost energy source. It could be the auto manufacturers are just prparing for that eventuality.

AC Transit in Berkeley, Oakland and other parts of the East Bay is running a pilot program with hydrogen fuel cells. Currently they have 12 buses running. They're great buses -- quiet and efficient. They have solar energy stations for making hydrogen, though they also use methane from landfills. Making hydrogen from electrolysis is a great way to use excess solar and wind energy.

I think it would be great to have a fully-electric car like a Tesla with enough battery capacity for local use -- say 100 miles / 160 km. And then be able to rent a small, plug-in fuel cell for long trips. That way you get rapid refueling and still get all the energy you need from wind and solar.

Storing H2 as either a high pressure gas or as a very cold liquid just does not make economic sense. As a high pressure gas, it is difficult to store enough gas for any range, and the pressure vessal is very heavy or expensive for the amount of H2 gas stored. As a liquid, the density improves, but the cost to produce the liquid is excessive in terms of energy.

The best method of storing hydrogen is in the form of NH3 which is even higher density than liquid H2, and only requires a low pressure tank to maintain it in liquid form. Even better, NH3 can be shipped through existing pipe transport systems and by tanker ship from overseas.

With the efficiency of shale-gas production in U.S., I am getting worried over these alternative green vehicles. With the deposit of shale gas in U.S., automakers should have a strategy to continue to build mileage optimality in the typical internal cumbustion engine. This is not going away anytime soon if we continue to have shale gas here. So, venturing into hydrogen and the likes may be a business model after the shale gas is done.

My bet is, since the natural gas burning engines you're talking about are ICEs, that the 35-40 percent figure is only valid when the engine is running at max efficiency, if valid at all. Meaning, with internal combustion, at high power.

Internal combustion uses the Carnot cycle, which describes heat engines. Its efficiency is based entirely on the difference between the combustion temperature and the exhaust temperature. The hotter the exhaust, the more energy is being wasted. To make the exhaust as cool as possible, you need the highest possible compression ratio (so that the expanding gas is providing power until it cools as much as possible, as it expands).

Working against you is that compression can only go so high before you experience premature detonation, and you need it to rev to create horsepower from torque. High revs require short stroke. So those two effects, premature detonation and the need for revs, limit how much efficiency can be derived from ICEs. So any claims I hear about 75 percent or more, which you get from time to time, are pure fiction.

On the other hand, reforming the fuel and feeding H2 to a fuel cell works on entirely different principles. And it looks like you should easily beat the ICE, in most actual driving, handily. Of course, there's cost to be worried about.

Steam reformers at gas stations sounds like a pretty good idea, actually, although you still need to store H2 in a pressurized tank in the car. It would really be nice to avoid that. Fun to watch this stuff develop.

Fuel cell cars which run on pure H2, when you take into account the fuel cell efficiency and the efficiency of electric motors, are roughly 65 perecent efficient or so. Although fuel cells are less efficient at their highest output. Still, something over 60 percent overall is the figure.

So the entire process is right about 50 percent efficiency.

That's interesting because a car buring natural gas directly is about 35-40% efficient. So you are saying that converting it to hydrogen and then converting the hydrogen to electricity to power the electric motors is more efficient than just buring the natural gas directly in the car.

And searching through the net, I've seen that apparently they can build these steam reformers to convert natural gas to hydrogen pretty small--small enough to be at a gas station. That would solve the distribution problem. Apparently Honda is even working on a Home Energy Station to create H2 at your house.

It is starting to look interesting. But we are probably many years away from this being widespread. Still, if I had a Tesla (I wish), it would certainly be nice if I could fill up a tank with H2 to power the electric motors.

If some of these metal-air batteries were ever to come to market, especially lithium-air, then the recharge rate would not matter. If you could get ten times the energy-density of lithium-ion this would imply ten times the range which would be over 2000 miles if we use the Tesla S as an example. That kind of range would be far in excess of a day's worth of driving.

Perhaps the road to better batteries is bumpier then they thought? Still it's hard to believe that hydrogen fuel-cell cars are a viable replacement. Ten years ago those fuel-cell cars were million dollar vehicles and there was no clear path to affordability. What's changed?

Agreed on regenerative braking. But all that requires is a mild hybrid type of setup, where the battery recoups that braking energy. The battery is not the primary energy storage system. Such an arrangement can certainly be used in fuel cell cars too.